The shrinking of a plastic foil can be accomplished with hand-held apparatus or by means of stationary automatically operating installations, e.g. as described in my copending application Ser. No. 566,330 filed Dec. 28, 1983. For good results of this operation, it is important to produce a uniform, flame-free stream of hot gas which can be obtained by providing the widest possible outflow area for the gases leaving the burner. For this purpose the combustion chamber of the burner and its discharge end should have an elongate shape. A flame retainer provides a uniform pressure distribution of the gases entering the combustion chamber and to improve the mixing of the combustible gases with the oxygen-containing gases.
A burner of the defined kind, e.g. is described in German utility model No. 82 20 335, has a flame holder in the shape of a transverse tube extending completely across the burner. The tube splits the gas flow entering the chamber into partial flows passing through the passages between the tube and the adjacent walls into the combustion chamber which are drawn toward its center by the dead-air space downstream of the tube, thereby lessening the strain on the combustion-chamber walls. The gases exit from the burner at a state of high turbulence, which leads to a reduction of the jet velocity of the hot gases at the burner exit and to a lower noise level.
For use with a foil-shrinking apparatus, however, it is desirable to have the gases leave the burner at high speed in order to let the stream entrain more cold ambient air which mixes with the hot gases and lowers their temperature, thus avoiding burns of the plastic foils to be shrunk.